The invention relates to a novel system for decoding high density barcode from a record medium at high rate of speed and more particularly to method and circuitry for special preprocessing of the high speed barcode data stream that is much too fast for a conventional microprocessor, and then completing decoding with such a microprocessor without loss of throughput speed.
As known, barcodes contain information encoded by the ratios of the widths and sequence of the black bars and intervening white spaces. The code may be read by rapidly sweeping a spot of light, with a rotating mirror, across the entire barcode pattern. Reflection of light from spaces and absorption at black bars are sensed by the scanner's photodetector causing a varying level signal to be inputted to the decoder. The relative signal level thus fluctuates with the bars and spaces and is representative of the coded bar pattern. Moving beam barcode scanning of high density barcodes can generate serial data streams containing pulses as short as 1 microsecond.
To decode such a high density barcode data stream, the time duration of all the bars and spaces (as represented by signal amplitudes) must be effectively measured and compared to valid code patterns. By contrast, in slow speed decoders, a conventional microprocessor is itself programmed to measure each bar and space, with its own relatively slow internal counters and store that digital representation of the bar/space width in memory. The ratios of adjacent bars and spaces are calculated by dividing one bar count by the other to obtain a series of ratios that comprise a valid coded character.
Using this technique, minimum bar times of about 12 microseconds are the approximate lower limit using a fairly fast 16 bit micro processor. However, this is an order of magnitude too slow for high speed moving beam scanners. Faster processors are available in the form of bit slice systems and sophisticated Digital Signal Processing chips; however, the cost and component count is prohibitive for many applications. Prior art decoders have employed hardware counters and DMA (direct memory access) controllers to measure the bars and spaces and put them directly into the microprocessor memory thus allowing higher scanning speeds. The problem with these known decoders is the excessive amount of hardware required, lack of adequate count range (without doing 16 bit data manipulation which is slow), and the presence of large amounts of extraneous background data in memory.
As described below, one of the features of the invention is to compress the input bar and space counts in a unique manner using a special log conversion of raw count information. One prior barcoding technique disclosed in U.S. Pat. No. 3,887,792 and also in U.S. Pat. No. 3,866,024 employs a logarithmic time base. By this prior art logarithmic technique, a reader can use the same scanner to decode from a high speed laser beam or a manually operated hand held wand. The technique disclosed in the above mentioned U.S. patents is a logarithmic technique but has drawbacks in not smoothly changing the log relationship and the time generator developed limits the decoder to a logarithmic base of two. Another prior art patent, U.S. Pat. No. 4,104,514 describes tailoring the logarithmic operation not limited to the base of 2 but does not effectively and optimally convert high density barcodes scanned by rapid beam scanners. Examples of other barcode decoders are disclosed in U.S. Pat. Nos. 4,282,426; 4,354,101 and 4,438,327.